A.C. Kruseman

VEPFIT applied to depth profiling problems

Abstract The modelling and fitting program VEPFIT has been employed in recent years for resolving defect depth profiles and depth structures of deposited layers. Recent activities concerning program development include the testing of a new model of MOS systems for implementation into VEPFIT and a study into decomposition of Doppler-broadened photo-peaks. Further methods are proposed using VEPFIT for analysis of lifetime measurements and for modelling of positron transport with multi-energy groups.

Two-detector Doppler broadening study of enhancement in Al

Enhancement due to positron-electron correlation has been studied in Al by calculating and measuring Doppler profiles and positron lifetimes. It is found that only the local density enhancement of Puska, Seitsonen and Nieminen can simultaneously reproduce the profile and the lifetime, but our analysis suggests that a GGA-type enhancement of reduced strength would likely give even better results.

Oxygen implanted silicon investigated by positron annihilation spectroscopy

Abstract In the manufacture of SIMOX a Si wafer is implanted with 1.7×10 18 , 180 keV oxygen ions. After implantation the wafer is annealed at 1350°C. This process creates a top layer of silicon, which almost has the quality of bulk silicon, and an insulating layer of buried oxide (BOX), that separates the top layer from the Si wafer. Positron annihilation techniques are very sensitive for the open-volume defects produced during the implantation. The Doppler broadening of the annihilation radiation technique was applied to investigate the effect of dose variations and anneal temperature. The positron annihilation results show that the top silicon layer in SIMOX still contains small vacancy–oxygen clusters which cannot be observed with TEM.

Positron beam analysis of semiconductor materials using a two-detector Doppler broadening coincidence system

Doppler-broadening measurements can be improved by using a second Ge-detector for the coincidence detection of the second annihilation photon. The coincidence condition in combination with an energy relation results in a reduction of the background by a factor of 100 compared to a single detector system. This background reduction opens up the possibility of performing accurate measurements of the high momentum part of Doppler-broadened annihilation spectra. We have used this technique for the analysis of a metal oxide semiconductor system and electron-irradiated, As- and Sb-doped silicon.

Buried oxide and defects in oxygen implanted Si monitored by positron annihilation

One- and two-detector Doppler broadening measurements performed on low (∼1014 to 1015 O+/cm2) and high dose (∼1017 to 1018 O+/cm2) oxygen-irradiated Si using variable-energy slow positrons are analyzed in terms of S and W parameters. After annealing the low-dose samples at 800 °C, large VxOy complexes are formed at depths around 400 nm. These complexes produce a clear-cut signature when the ratio of S to that of defect-free bulk Si is plotted. Similar behavior is found for samples irradiated with 2 and 4×1017 O+/cm2 and annealed at 1000 °C. After irradiation with 1.7×1018 O+/cm2 and anneal at 1350 °C a 170 nm thick almost-bulk-quality Si surface layer is formed on top of a 430 nm thick buried oxide layer. This method of preparation is called separation by implantation of oxygen. S−W measurements show that the surface layer contains electrically inactive VxOy complexes not seen by electron microscopy. A method is presented to decompose the Doppler broadening line shape into contributions of the bulk, surfa...

The momentum distribution of annihilating positron-electron pairs in aluminum

Experimental and theoretical results for the momentum distribution of the positron-electron pairs in defect-free bulk Al and Al containing vacancies are presented. The experimental data was obtained using a two-Ge-detector coincidence measurement of the Doppler broadening of the annihilation radiation. This setup enables an accurate measurement of the high-momentum part of the spectrum, thereby making it possible to compare experimental and theoretical results over a wider momentum range. Various approximations of the localized positron wave function (in the presence of an ideal monovacancy) are discussed.

Investigation of vacancies in GaN by positron annihilation

Positron beam analysis has been performed on autodoped n-type, semi-insulating and Mg doped p-type epitaxially grown layers of GaN on sapphire. Doppler broadening measurements clearly indicate the presence of vacancies in the intrinsically autodoped n-type GaN by an increase in the annihilation Doppler lineshape S-parameter of 1.04 relative to the value for the high resistivity sample. This value is typical for vacancy-type defects in compound semiconductors such as GaAs. Results of experiments with higher sensitivity to core-electrons are also presented. These two detector coincidence measurements yield information on the chemical environment surrounding the vacancies. The results are consistent with the presence of Ga vacancies in the autodoped n-type sample.

Positron beam investigations of natural cubic and coated diamonds

Abstract Positron beam and 2D-ACAR investigation of cubic and coated diamonds are reported. In type IIA diamonds, positrons are mostly trapped in vacancies in the carbon lattice; in type Ia diamonds, two main defect-related annihilation sites are nitrogen-vacancy complexes (H2, H3) and the vicinity of split interstitial atoms. No correlation between principal nitrogen defects and annihilation rate was found. PAS data indicate the presence of a significant amount of vacancies in all studied diamonds, which increases the rate of nitrogen aggregation. It is shown that pressurised fluid inclusions may serve as a positron trap, giving rise to the long component in the lifetime spectra.

The effect of the detector resolution on the Doppler broadening measurements of both valence and core electron–positron annihilation

Abstract The measured value of the Doppler profile of the electron–positron annihilation radiation depends on the resolution function of the detectors used in the experiment. This is illustrated by the results of calculations of the profile convoluted with resolution functions of different FWHM. These results are compared with Doppler broadening measurements for Al. The calculated results agree qualitatively with the experimental data. The positions of the peaks in the curves which give the ratio of a Doppler profile to a reference profile are found to shift towards higher momenta when the width of the resolution for the reference material was increased. Since the peaks in the ratio curves are used for identifying the chemical species, it is important that the peak positions be unambiguously characterized. Hence, the detector resolution should be carefully measured and quoted when reporting and comparing results of Doppler profiles.